Wellness & Medicine

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Unexpected Events Bring Lost Fear Memories Back

Fearful memories can be dampened by imagining past traumas in a safe setting. The "extinction" of fear is fragile, however, and surprising or unexpected events can cause fear memories to return. Inactivating brain areas that detect novelty prevents relapse of unwanted fear memories.

Traumatic and emotional experiences often lead to debilitating mental health disorders, including post-traumatic stress disorder (PTSD).  In the clinic, it is typical to use behvioral therapies such as exposure therapy to help reduce fear in patients suffering from traumatic memories.  Using these approaches, patients are asked to remember the circumstances and stimuli surrounding their tramuatic memory in a safe setting in order to "extinguish" their fear response to those events.  While effective in many cases, the loss of fear and anxiety achieved by these therapies is often short-lived—fear returns or relapses under a variety of conditions.  

Many years ago, the famous Russian physiologist Ivan Pavlov noted that simply exposing animals to novel or unexpected events could cause extinguished responses (such as salivary responses to sounds) to return.  Might exposure to novelty also cause extinguished fear responses to return?  

In a recent study (Maren, 2014), rats first learned that an innocuous tone predicted an aversive (but mild) electric shock to their feet.  The subsequent fear response to the tone was then extinguished by presenting the stimulus to the animals many times without the shock.  After the fear response to the tone was reduced with the extincttion procedure, they were then presented with the tone in either a new location (a novel test box) or in a familiar location, but in the presence of an unexpected sound (a noise burst).  In both cases, fear to the tone returned as Pavlov predicted:  the unexpected places and sounds led to a disinhibition of fear—in other words, fear relapsed.  

It turns out that parts of the brain important for recognition memory, including the hippocampus, are important mediators of relapse.  Temporarily inactivating the hippocampus in rats before the memory tests prevented the return of fear.  Ultimately, this suggests that patients undergoing behavioral therapies for anxiety disorders might also be helped by being better prepared to "expect the unexpected".  While it is not likely that manipulations that dampen the function of memory systems would be recommended treatments, it is possible that developing skills for coping with novelty and change might reduce the fear of the unexpected that often drives relapse.  Stephen Maren, PhD 7/17/2014

 

Original Article (2014)

Funded by: National Institutes of Health (R01MH065961)

Written by: Stephen Maren.

Link to journal article